I would think that most any small ceramic or aluminum cased power resistor would work just fine. For control I would epoxy the thermistor right to the resistor then bury the whole thing in sand or what ever insulating material you want to use.
I once used a few SMT resistors on a flex PC cable with a thermistor in a high gain closed loop to get a 40'C rise regulated within 0.1'C in 20 seconds. with 5mm of foam insulation around a low mass crystal can. Your mass will take longer, but low conductivity to target and high insulation to ambient is essential for stability.
I would think that most any small ceramic or aluminum cased power resistor would work just fine. For control I would epoxy the thermistor right to the resistor then bury the whole thing in sand or what ever insulating material you want to use.
Sorry I didn't see yours up to now. The smaller the better. Otherwise you will have a sensor taking longer time than the sample, to cool down or warm up.
My very first project with I micro was a temperature display of up to 32 sensors for a greenhouse. The sensors (LM135 IIRC) went inside a jacket (?) of thick brass pipe and worked quite well but trying to get the temperature of my armpit took forever until the cold things warmed up and showed the right value.